Hernia is the protrusion of an organ, or part of an organ, from its proper cavity, whether as the result of a rent in the tissues, as when the belly is broken, or an escape through a natural or imperfectly-closed orifice, as in inguinal and umbilical hernia.

It is convenient to divide ruptures into two classes, viz., congenital and acquired: in the one the defect is present at the time of birth; in the other it is brought about afterwards, by accident or disease. The causes of this latter form of the malady are severe straining, as in heavy draught, jumping, rearing, rolling, and kicking, or it may follow upon some form of external violence, as kicks from other horses, horning by cattle, or blows inflicted upon the abdomen in various other ways.

Hernia is described as reducible and irreducible. It is reducible when it can be returned into its proper place by simple external manipulation or taxis. It is irreducible when its return cannot be effected without a surgical operation.

A rupture is said to be strangulated when the orifice through which it passes becomes too small to accommodate the extruded organ without imparting to it considerable pressure, in which case the circulation is interfered with, the blood gathers in the vessels of the escaped viscus, and sooner or later provokes swelling and inflammation in it, which may end in mortification and death of the part.